Low overhead resynchronization snapshot creation and utilization

ABSTRACT

One or more techniques and/or computing devices are provided for resynchronization. For example, a request may be received to create pseudo snapshots of a first consistency group, hosted by a first storage controller, and a second consistency group, hosted by a second storage controller, having a synchronous replication relationship with the first consistency group. Incoming client write requests are logged within an intercept tracking log at the first storage controller. After a first drain without hold of incoming write requests is performed, a first pseudo common snapshot of the second consistency group is created. After a second drain without hold of incoming write operations is performed, a second pseudo common snapshot of the first consistency group and the intercept tracking log is created. The pseudo snapshots and the intercept tracking log (e.g., indicating a delta between the pseudo snapshots) are used to resynchronize the first and second consistency groups.

BACKGROUND

Many storage networks may implement data replication and/or otherredundancy data access techniques for data loss protection andnon-disruptive client access. For example, a first storage cluster maycomprise a first storage controller configured to provide clients withprimary access to data stored within a first storage device and/or otherstorage devices. A second storage cluster may comprise a second storagecontroller configured to provide clients with primary access to datastored within a second storage device and/or other storage devices. Thefirst storage controller and the second storage controller may beconfigured according to a disaster recovery relationship, such that thesecond storage controller may provide failover access to replicated datathat was replicated from the first storage device to a secondary storagedevice, owned by the first storage controller, but accessible to thesecond storage controller (e.g., a switchover operation may be performedwhere the second storage controller assumes ownership of the secondarystorage device and/or other storage devices previously owned by thefirst storage controller so that the second storage controller mayprovide clients with failover access to replicated data within suchstorage devices).

In an example, the second storage cluster may be located at a remotesite to the first storage cluster (e.g., storage clusters may be locatedin different buildings, cities, thousands of kilometers from oneanother, etc.). Thus, if a disaster occurs at a site of a storagecluster, then a surviving storage cluster may remain unaffected by thedisaster (e.g., a power outage of a building hosting the first storagecluster may not affect a second building hosting the second storagecluster in a different city).

In an example, two storage controllers within a storage cluster may beconfigured according to a high availability configuration, such as wherethe two storage controllers are locally connected to one another and/orto the same storage devices. In this way, when a storage controllerfails, then a high availability partner storage controller can quicklytakeover for the failed storage controller due to the localconnectivity. Thus, the high availability partner storage controller mayprovide clients with access to data previously accessible through thefailed storage controller.

Various replication and synchronization techniques may be used toreplicate data (e.g., client data), configuration data (e.g., a size ofa volume, a name of a volume, etc.), and/or write caching data (e.g.,cached write operations) between storage controllers and/or storagedevices. In an example of synchronization, a synchronous replicationrelationship may be implemented between the first storage controller andthe second storage controller, such that an incoming write operation tothe first storage controller is locally implemented upon a firstconsistency group (e.g., one or more files, logical unit number (LUNs),LUNs spanning multiple volumes, or any other type of storage object) bythe first storage controller and remotely implemented upon a secondconsistency group (e.g., maintained as a backup replication of the firstconsistency group) by the second storage controller before anacknowledgement is provided back to a client that sent the incomingwrite operation. In an example of replication, snapshots of the firstconsistency group may be used to replicate the first consistency groupto the second consistency group. For example, a base snapshot of thefirst consistency group (e.g., a volume comprising the first consistencygroup) may be used to initially create the second consistency group. Acurrent incremental snapshot of the first consistency group (e.g., thevolume) may be used to replicate changes made to the first consistencygroup since the base snapshot or since a last incremental snapshot.Snapshots may also be periodically created and used to recover fromoperational failures or corruption. Unfortunately, snapshot creation maybe disruptive to client access to the first consistency group (e.g.,client write requests may be blocked during snapshot creation) and/ormay be disruptive to the synchronous replication relationship (e.g., ifclient write operations are not blocked and are implemented upon thefirst consistency group while a snapshot of the second consistency groupis being created, then data divergence between the first consistencygroup and the second consistency group can occur). For example, clientwrite requests to the first consistency group may be rejected duringsnapshot creation, thus increasing latency and client data accessdisruption.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a component block diagram illustrating an example clusterednetwork in accordance with one or more of the provisions set forthherein.

FIG. 2 is a component block diagram illustrating an example data storagesystem in accordance with one or more of the provisions set forthherein.

FIG. 3 is a flow chart illustrating an exemplary method of pseudo commonsnapshot creation and utilization.

FIG. 4A is a component block diagram illustrating an exemplary computingdevice for pseudo common snapshot creation, where a write operation islocally implemented by a first storage controller and remotelyimplemented by a second storage controller based upon a synchronousreplication relationship.

FIG. 4B is a component block diagram illustrating an exemplary computingdevice for pseudo common snapshot creation, where a request to createpseudo common snapshots is received.

FIG. 4C is a component block diagram illustrating an exemplary computingdevice for pseudo common snapshot creation, where incoming client writerequests are logged.

FIG. 4D is a component block diagram illustrating an exemplary computingdevice for pseudo common snapshot creation, where a first drain withouthold is performed.

FIG. 4E is a component block diagram illustrating an exemplary computingdevice for pseudo common snapshot creation, where a first pseudo commonsnapshot is created.

FIG. 4F is a component block diagram illustrating an exemplary computingdevice for pseudo common snapshot creation, where a second drain withouthold is performed.

FIG. 4G is a component block diagram illustrating an exemplary computingdevice for pseudo common snapshot creation, where a second pseudo commonsnapshot is created.

FIG. 4H is a component block diagram illustrating an exemplary computingdevice for pseudo common snapshot creation, where an intercept trackinglog is deleted.

FIG. 5A is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots.

FIG. 5B is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots, where asynchronous replication relationship goes out-of-sync.

FIG. 5C is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots, where alocal rollback base snapshot is created.

FIG. 5D is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots, where anactive file system is rolled back.

FIG. 5E is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots, where datadifferences are applied to a second consistency group.

FIG. 5F is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots, where asyncreplication is performed.

FIG. 6A is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots.

FIG. 6B is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots, where aswitchover operation is performed.

FIG. 6C is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots, asynchronous replication relationship transitions into an out-of-syncstate.

FIG. 6D is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots, where alocal rollback base snapshot is created.

FIG. 6E is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots, where anactive file system is rolled back.

FIG. 6F is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots, where afirst consistency group is restored.

FIG. 6G is a component block diagram illustrating an exemplary computingdevice for resynchronization using pseudo common snapshots, where asyncreplication is performed.

FIG. 7 is an example of a computer readable medium in accordance withone or more of the provisions set forth herein.

DETAILED DESCRIPTION

Some examples of the claimed subject matter are now described withreference to the drawings, where like reference numerals are generallyused to refer to like elements throughout. In the following description,for purposes of explanation, numerous specific details are set forth inorder to provide an understanding of the claimed subject matter. It maybe evident, however, that the claimed subject matter may be practicedwithout these specific details. Nothing in this detailed description isadmitted as prior art.

One or more techniques and/or computing devices for pseudo commonsnapshot creation and utilization are provided herein. For example, asynchronous replication relationship may exist between a firstconsistency group hosted by a first storage controller and a secondconsistency group hosted by a second storage controller (e.g., one ormore files, LUNs, LUNs stored across multiple volumes, volumes,subdirectories, or any other storage objects may be synchronouslyreplicated between the storage controllers, such that modifications tothe first consistency group are replicated to the second consistencygroup before acknowledgements are provided back to clients requestingsuch modifications). A request to create pseudo common snapshots of thefirst consistency group and the second consistency group may bereceived. An intercept tracking log may be used to record datadifferences between a first pseudo command snapshot of the secondconsistency group and a subsequently created second pseudo commonsnapshot of the first consistency group. The pseudo common snapshots arecreated in a non-disruptive manner where incoming client write requestsare not paused during snapshot creation and are still split andreplicated between the first consistency group and the secondconsistency group. In this way, disruption of client data access to thefirst consistency group is reduced, and the synchronous replicationrelationship is maintained.

If the second storage controller falls behind (e.g., new client writeoperations are committed by the first storage controller but are notreplicated to the second storage controller such as due to a networkissue) and the synchronous replication relationship becomes out-of-sync,then the pseudo common snapshots may be used to perform a forwardresynchronization to bring the second consistency group of the secondstorage controller in sync with the first consistency group. If thefirst storage controller falls behind due to a switchover operation anda subsequent switchback operation (e.g., the first storage controllermay fail, and thus the second storage controller may perform aswitchover operation to obtain ownership of storage devices previouslyowned by the first storage controller for providing clients withfailover access to data within the storage devices, and then a switchback to the first storage controller for providing clients with primaryaccess to the data may be performed after the first storage controllerrecovers) and the synchronous replication relationship becomesout-of-sync, then the pseudo common snapshots may be used to perform areverse resynchronization. After the forward or reverseresynchronization is complete, other synchronization techniques (e.g.,async replication using snapshots and incremental transfers) may beperformed.

To provide context for pseudo common snapshot creation and utilization,FIG. 1 illustrates an embodiment of a clustered network environment 100or a network storage environment. It may be appreciated, however, thatthe techniques, etc. described herein may be implemented within theclustered network environment 100, a non-cluster network environment,and/or a variety of other computing environments, such as a desktopcomputing environment. That is, the instant disclosure, including thescope of the appended claims, is not meant to be limited to the examplesprovided herein. It will be appreciated that where the same or similarcomponents, elements, features, items, modules, etc. are illustrated inlater figures but were previously discussed with regard to priorfigures, that a similar (e.g., redundant) discussion of the same may beomitted when describing the subsequent figures (e.g., for purposes ofsimplicity and ease of understanding).

FIG. 1 is a block diagram illustrating the clustered network environment100 that may implement at least some embodiments of the techniquesand/or systems described herein. The clustered network environment 100comprises data storage systems 102 and 104 that are coupled over acluster fabric 106, such as a computing network embodied as a privateInfiniband, Fibre Channel (FC), or Ethernet network facilitatingcommunication between the data storage systems 102 and 104 (and one ormore modules, component, etc. therein, such as, nodes 116 and 118, forexample). It will be appreciated that while two data storage systems 102and 104 and two nodes 116 and 118 are illustrated in FIG. 1, that anysuitable number of such components is contemplated. In an example, nodes116, 118 comprise storage controllers (e.g., node 116 may comprise aprimary or local storage controller and node 118 may comprise asecondary or remote storage controller) that provide client devices,such as host devices 108, 110, with access to data stored within datastorage devices 128, 130. Similarly, unless specifically providedotherwise herein, the same is true for other modules, elements,features, items, etc. referenced herein and/or illustrated in theaccompanying drawings. That is, a particular number of components,modules, elements, features, items, etc. disclosed herein is not meantto be interpreted in a limiting manner.

It will be further appreciated that clustered networks are not limitedto any particular geographic areas and can be clustered locally and/orremotely. Thus, in one embodiment a clustered network can be distributedover a plurality of storage systems and/or nodes located in a pluralityof geographic locations; while in another embodiment a clustered networkcan include data storage systems (e.g., 102, 104) residing in a samegeographic location (e.g., in a single onsite rack of data storagedevices).

In the illustrated example, one or more host devices 108, 110 which maycomprise, for example, client devices, personal computers (PCs),computing devices used for storage (e.g., storage servers), and othercomputers or peripheral devices (e.g., printers), are coupled to therespective data storage systems 102, 104 by storage network connections112, 114. Network connection may comprise a local area network (LAN) orwide area network (WAN), for example, that utilizes Network AttachedStorage (NAS) protocols, such as a Common Internet File System (CIFS)protocol or a Network File System (NFS) protocol to exchange datapackets. Illustratively, the host devices 108, 110 may begeneral-purpose computers running applications, and may interact withthe data storage systems 102, 104 using a client/server model forexchange of information. That is, the host device may request data fromthe data storage system (e.g., data on a storage device managed by anetwork storage control configured to process I/O commands issued by thehost device for the storage device), and the data storage system mayreturn results of the request to the host device via one or more storagenetwork connections 112, 114.

The nodes 116, 118 on clustered data storage systems 102, 104 cancomprise network or host nodes that are interconnected as a cluster toprovide data storage and management services, such as to an enterprisehaving remote locations, cloud storage (e.g., a storage endpoint may bestored within a data cloud), etc., for example. Such a node in theclustered network environment 100 can be a device attached to thenetwork as a connection point, redistribution point or communicationendpoint, for example. A node may be capable of sending, receiving,and/or forwarding information over a network communications channel, andcould comprise any device that meets any or all of these criteria. Oneexample of a node may be a data storage and management server attachedto a network, where the server can comprise a general purpose computeror a computing device particularly configured to operate as a server ina data storage and management system.

In an example, a first cluster of nodes such as the nodes 116, 118(e.g., a first set of storage controllers configured to provide accessto a first storage aggregate comprising a first logical grouping of oneor more storage devices) may be located on a first storage site. Asecond cluster of nodes, not illustrated, may be located at a secondstorage site (e.g., a second set of storage controllers configured toprovide access to a second storage aggregate comprising a second logicalgrouping of one or more storage devices). The first cluster of nodes andthe second cluster of nodes may be configured according to a disasterrecovery configuration where a surviving cluster of nodes providesswitchover access to storage devices of a disaster cluster of nodes inthe event a disaster occurs at a disaster storage site comprising thedisaster cluster of nodes (e.g., the first cluster of nodes providesclient devices with switchover data access to storage devices of thesecond storage aggregate in the event a disaster occurs at the secondstorage site).

As illustrated in the clustered network environment 100, nodes 116, 118can comprise various functional components that coordinate to providedistributed storage architecture for the cluster. For example, the nodescan comprise network modules 120, 122 and data modules 124, 126. Networkmoduless 120, 122 can be configured to allow the nodes 116, 118 (e.g.,network storage controllers) to connect with host devices 108, 110 overthe storage network connections 112, 114, for example, allowing the hostdevices 108, 110 to access data stored in the distributed storagesystem. Further, the network modules 120, 122 can provide connectionswith one or more other components through the cluster fabric 106. Forexample, in FIG. 1, the network module 120 of node 116 can access asecond data storage device 130 by sending a request through the datamodule 126 of a second node 118.

Data modules 124, 126 can be configured to connect one or more datastorage devices 128, 130, such as disks or arrays of disks, flashmemory, or some other form of data storage, to the nodes 116, 118. Thenodes 116, 118 can be interconnected by the cluster fabric 106, forexample, allowing respective nodes in the cluster to access data on datastorage devices 128, 130 connected to different nodes in the cluster.Often, data modules 124, 126 communicate with the data storage devices128, 130 according to a storage area network (SAN) protocol, such asSmall Computer System Interface (SCSI) or Fiber Channel Protocol (FCP),for example. Thus, as seen from an operating system on nodes 116, 118,the data storage devices 128, 130 can appear as locally attached to theoperating system. In this manner, different nodes 116, 118, etc. mayaccess data blocks through the operating system, rather than expresslyrequesting abstract files.

It should be appreciated that, while the clustered network environment100 illustrates an equal number of network and data modules, otherembodiments may comprise a differing number of these modules. Forexample, there may be a plurality of network and data modulesinterconnected in a cluster that does not have a one-to-onecorrespondence between the network and data modules. That is, differentnodes can have a different number of network and data modules, and thesame node can have a different number of network modules than datamodules.

Further, a host device 108, 110 can be networked with the nodes 116, 118in the cluster, over the storage networking connections 112, 114. As anexample, respective host devices 108, 110 that are networked to acluster may request services (e.g., exchanging of information in theform of data packets) of nodes 116, 118 in the cluster, and the nodes116, 118 can return results of the requested services to the hostdevices 108, 110. In one embodiment, the host devices 108, 110 canexchange information with the network modules 120, 122 residing in thenodes 116, 118 (e.g., network hosts) in the data storage systems 102,104.

In one embodiment, the data storage devices 128, 130 comprise volumes132, which is an implementation of storage of information onto diskdrives or disk arrays or other storage (e.g., flash) as a file-systemfor data, for example. Volumes can span a portion of a disk, acollection of disks, or portions of disks, for example, and typicallydefine an overall logical arrangement of file storage on disk space inthe storage system. In one embodiment a volume can comprise stored dataas one or more files that reside in a hierarchical directory structurewithin the volume.

Volumes are typically configured in formats that may be associated withparticular storage systems, and respective volume formats typicallycomprise features that provide functionality to the volumes, such asproviding an ability for volumes to form clusters. For example, where afirst storage system may utilize a first format for their volumes, asecond storage system may utilize a second format for their volumes.

In the clustered network environment 100, the host devices 108, 110 canutilize the data storage systems 102, 104 to store and retrieve datafrom the volumes 132. In this embodiment, for example, the host device108 can send data packets to the network module 120 in the node 116within data storage system 102. The node 116 can forward the data to thedata storage device 128 using the data module 124, where the datastorage device 128 comprises volume 132A. In this way, in this example,the host device can access the volume 132A, to store and/or retrievedata, using the data storage system 102 connected by the networkconnection 112. Further, in this embodiment, the host device 110 canexchange data with the network module 122 in the host 118 within thedata storage system 104 (e.g., which may be remote from the data storagesystem 102). The host 118 can forward the data to the data storagedevice 130 using the data module 126, thereby accessing volume 132Bassociated with the data storage device 130.

It may be appreciated that pseudo common snapshot creation andutilization may be implemented within the clustered network environment100. In an example, a synchronous replication relationship may beestablished between the volume 132A (e.g., a first consistency group ofthe volume 132A, such as a subset of the volume 132A comprising one ormore files, LUNs, or other storage objects) of node 116 (e.g., a firststorage controller) and the volume 132B (e.g., a second consistencygroup of the volume 132B, such as a subset of the volume 132B comprisingone or more files, LUNs, or other storage objects) of the node 118(e.g., a second storage controller). Pseudo common snapshots of thevolume 132A and the volume 132B may be created in a non-disruptivemanner where incoming client write requests are not paused duringsnapshot creation and are still split and replicated between the volume132A and the volume 132B. It may be appreciated that snapshot creationand utilization may be implemented for and/or between any type ofcomputing environment, and may be transferrable between physical devices(e.g., node 116, node 118, a desktop computer, a tablet, a laptop, awearable device, a mobile device, a storage device, a server, etc.)and/or a cloud computing environment (e.g., remote to the clusterednetwork environment 100).

FIG. 2 is an illustrative example of a data storage system 200 (e.g.,102, 104 in FIG. 1), providing further detail of an embodiment ofcomponents that may implement one or more of the techniques and/orsystems described herein. The data storage system 200 comprises a node202 (e.g., host nodes 116, 118 in FIG. 1), and a data storage device 234(e.g., data storage devices 128, 130 in FIG. 1). The node 202 may be ageneral purpose computer, for example, or some other computing deviceparticularly configured to operate as a storage server. A host device205 (e.g., 108, 110 in FIG. 1) can be connected to the node 202 over anetwork 216, for example, to provides access to files and/or other datastored on the data storage device 234. In an example, the node 202comprises a storage controller that provides client devices, such as thehost device 205, with access to data stored within data storage device234.

The data storage device 234 can comprise mass storage devices, such asdisks 224, 226, 228 of a disk array 218, 220, 222. It will beappreciated that the techniques and systems, described herein, are notlimited by the example embodiment. For example, disks 224, 226, 228 maycomprise any type of mass storage devices, including but not limited tomagnetic disk drives, flash memory, and any other similar media adaptedto store information, including, for example, data (D) and/or parity (P)information.

The node 202 comprises one or more processors 204, a memory 206, anetwork adapter 210, a cluster access adapter 212, and a storage adapter214 interconnected by a system bus 242. The data storage system 200 alsoincludes an operating system 208 installed in the memory 206 of the node202 that can, for example, implement a Redundant Array of Independent(or Inexpensive) Disks (RAID) optimization technique to optimize areconstruction process of data of a failed disk in an array.

The operating system 208 can also manage communications for the datastorage system, and communications between other data storage systemsthat may be in a clustered network, such as attached to a cluster fabric215 (e.g., 106 in FIG. 1). Thus, the node 202, such as a network storagecontroller, can respond to host device requests to manage data on thedata storage device 234 (e.g., or additional clustered devices) inaccordance with these host device requests. The operating system 208 canoften establish one or more file systems on the data storage system 200,where a file system can include software code and data structures thatimplement a persistent hierarchical namespace of files and directories,for example. As an example, when a new data storage device (not shown)is added to a clustered network system, the operating system 208 isinformed where, in an existing directory tree, new files associated withthe new data storage device are to be stored. This is often referred toas “mounting” a file system.

In the example data storage system 200, memory 206 can include storagelocations that are addressable by the processors 204 and networkadapters 210, 212, 214 for storing related software application code anddata structures. The processors 204 and network adapters 210, 212, 214may, for example, include processing elements and/or logic circuitryconfigured to execute the software code and manipulate the datastructures. The operating system 208, portions of which are typicallyresident in the memory 206 and executed by the processing elements,functionally organizes the storage system by, among other things,invoking storage operations in support of a file service implemented bythe storage system. It will be apparent to those skilled in the art thatother processing and memory mechanisms, including various computerreadable media, may be used for storing and/or executing applicationinstructions pertaining to the techniques described herein. For example,the operating system can also utilize one or more control files (notshown) to aid in the provisioning of virtual machines.

The network adapter 210 includes the mechanical, electrical andsignaling circuitry needed to connect the data storage system 200 to ahost device 205 over a network 216, which may comprise, among otherthings, a point-to-point connection or a shared medium, such as a localarea network. The host device 205 (e.g., 108, 110 of FIG. 1) may be ageneral-purpose computer configured to execute applications. Asdescribed above, the host device 205 may interact with the data storagesystem 200 in accordance with a client/host model of informationdelivery.

The storage adapter 214 cooperates with the operating system 208executing on the node 202 to access information requested by the hostdevice 205 (e.g., access data on a storage device managed by a networkstorage controller). The information may be stored on any type ofattached array of writeable media such as magnetic disk drives, flashmemory, and/or any other similar media adapted to store information. Inthe example data storage system 200, the information can be stored indata blocks on the disks 224, 226, 228. The storage adapter 214 caninclude input/output (I/O) interface circuitry that couples to the disksover an I/O interconnect arrangement, such as a storage area network(SAN) protocol (e.g., Small Computer System Interface (SCSI), iSCSI,hyperSCSI, Fiber Channel Protocol (FCP)). The information is retrievedby the storage adapter 214 and, if necessary, processed by the one ormore processors 204 (or the storage adapter 214 itself) prior to beingforwarded over the system bus 242 to the network adapter 210 (and/or thecluster access adapter 212 if sending to another node in the cluster)where the information is formatted into a data packet and returned tothe host device 205 over the network 216 (and/or returned to anothernode attached to the cluster over the cluster fabric 215).

In one embodiment, storage of information on disk arrays 218, 220, 222can be implemented as one or more storage volumes 230, 232 that arecomprised of a cluster of disks 224, 226, 228 defining an overalllogical arrangement of disk space. The disks 224, 226, 228 that compriseone or more volumes are typically organized as one or more groups ofRAIDs. As an example, volume 230 comprises an aggregate of disk arrays218 and 220, which comprise the cluster of disks 224 and 226.

In one embodiment, to facilitate access to disks 224, 226, 228, theoperating system 208 may implement a file system (e.g., write anywherefile system) that logically organizes the information as a hierarchicalstructure of directories and files on the disks. In this embodiment,respective files may be implemented as a set of disk blocks configuredto store information, whereas directories may be implemented asspecially formatted files in which information about other files anddirectories are stored.

Whatever the underlying physical configuration within this data storagesystem 200, data can be stored as files within physical and/or virtualvolumes, which can be associated with respective volume identifiers,such as file system identifiers (FSIDs), which can be 32-bits in lengthin one example.

A physical volume corresponds to at least a portion of physical storagedevices whose address, addressable space, location, etc. doesn't change,such as at least some of one or more data storage devices 234 (e.g., aRedundant Array of Independent (or Inexpensive) Disks (RAID system)).Typically the location of the physical volume doesn't change in that the(range of) address(es) used to access it generally remains constant.

A virtual volume, in contrast, is stored over an aggregate of disparateportions of different physical storage devices. The virtual volume maybe a collection of different available portions of different physicalstorage device locations, such as some available space from each of thedisks 224, 226, and/or 228. It will be appreciated that since a virtualvolume is not “tied” to any one particular storage device, a virtualvolume can be said to include a layer of abstraction or virtualization,which allows it to be resized and/or flexible in some regards.

Further, a virtual volume can include one or more logical unit numbers(LUNs) 238, directories 236, Qtrees 235, and files 240. Among otherthings, these features, but more particularly LUNS, allow the disparatememory locations within which data is stored to be identified, forexample, and grouped as data storage unit. As such, the LUNs 238 may becharacterized as constituting a virtual disk or drive upon which datawithin the virtual volume is stored within the aggregate. For example,LUNs are often referred to as virtual drives, such that they emulate ahard drive from a general purpose computer, while they actually comprisedata blocks stored in various parts of a volume.

In one embodiment, one or more data storage devices 234 can have one ormore physical ports, wherein each physical port can be assigned a targetaddress (e.g., SCSI target address). To represent respective volumesstored on a data storage device, a target address on the data storagedevice can be used to identify one or more LUNs 238. Thus, for example,when the node 202 connects to a volume 230, 232 through the storageadapter 214, a connection between the node 202 and the one or more LUNs238 underlying the volume is created.

In one embodiment, respective target addresses can identify multipleLUNs, such that a target address can represent multiple volumes. The I/Ointerface, which can be implemented as circuitry and/or software in thestorage adapter 214 or as executable code residing in memory 206 andexecuted by the processors 204, for example, can connect to volume 230by using one or more addresses that identify the one or more LUNs 238.

It may be appreciated that pseudo common snapshot creation andutilization may be implemented for the data storage system 200. In anexample, a synchronous replication relationship may be establishedbetween the volume 230 (e.g., a first consistency group of the volume230, such as a subset of the volume 230 comprising one or more files,LUNs, or other storage objects) of the node 202 (e.g., a first storagecontroller) and a second volume (e.g., a second consistency group of thesecond volume, such as a subset of the second volume comprising one ormore files, LUNs, or other storage objects) of a second node (e.g., asecond storage controller). Pseudo common snapshots of the volume 230and the second volume may be created in a non-disruptive manner whereincoming client write request are not paused during snapshot creationand are still split and replicated between the volume 230 and the secondvolume. It may be appreciated that snapshot creation and utilization maybe implemented for and/or between any type of computing environment, andmay be transferrable between physical devices (e.g., node 202, hostdevice 205, a desktop computer, a tablet, a laptop, a wearable device, amobile device, a storage device, a server, etc.) and/or a cloudcomputing environment (e.g., remote to the node 202 and/or the hostdevice 205).

One embodiment of pseudo common snapshot creation and utilization isillustrated by an exemplary method 300 of FIG. 3. In an example, a firststorage controller may host a first consistency group, such as one ormore files, one or more LUNs, one or more LUNs spanning across multiplevolumes, a volume, a subdirectory within the volume, or any other typeof object used to store data. A second storage controller may host asecond consistency group that is a backup replication of the firstconsistency group. A synchronous replication relationship may beestablished between the first consistency group and the secondconsistency group, such that a modification to the first consistencygroup (e.g., a write operation from a client to a file) is replicated tothe second consistency group before an acknowledgement is provided backto the client, thus preserving data consistency between the firstconsistency group and the second consistency group.

In an example of the synchronous replication relationship, amodification request (e.g., a write operation) that is received by adata module of the first storage controller is intercepted and checkedto see if the modification request is to a file or LUN (e.g., the firstconsistency group) that is part of the synchronous replicationrelationship. If the modification request is to the file or LUN that ispart of the synchronous replication relationship, then the modificationrequest is intercepted and a first copy of the modification request issent to a primary file system of the first storage controller forimplementation, and a second copy of the modification request is sent tothe second storage controller for implementation (e.g., implementationupon the second consistency group). Once the modification request iscompleted by both the first storage controller and the second storagecontroller, then the modification request is acknowledged back to aclient that sent the modification request. If the modification requestsucceeds at the first storage controller but fails at the second storagecontroller, then the acknowledgment is sent to the client and thesynchronous replication relationship is transitioned into an out-of-syncstate. Synchronous replication may be provided for a consistency groupof files and/or LUNs using multiple splitters, such that a singlesplitter is used to split modification requests to a particular file orLUN and the splitter tracks a status of ongoing modification requests.In an example, one or more splitters, for the first consistency group(e.g., a splitter per file or LUN or a single splitter for files and/orLUNs of the first consistency group), is configured to split writeoperations to target both the first consistency group and the secondconsistency group. For example, the splitter may receive a writeoperation targeting a first file or LUN of the first consistency group.The splitter may split the write operation into a replication writeoperation. The splitter may instruct the first storage controller tolocally implement the write operation upon the first file or LUN andinstruct the second storage controller to remotely implement thereplication write operation upon a second file or LUN within the secondconsistency group.

At 302, a request to make pseudo common snapshots of the firstconsistency group may be received, such as by the first storagecontroller. At 304, the logging of incoming write requests to anintercept tracking log may be enabled at a first point in time (e.g., anepoch time may be change to a first epoch time). The intercept trackinglog may comprise indicators that can be set to indicate whether portionsof the first consistency group (e.g., a range of blocks or any othergranularity of the first consistency group) have not been modified(e.g., a clean region comprising the same data as a correspondingportion of the second consistency group that is a backup replication ofthe clean region) or have been modified (e.g., a dirty region that hasbeen modified by a client write request, and thus may potentiallycomprise different data than a corresponding region within the secondconsistency group that is a backup replication of the dirty region) withrespect to the second consistency group. In an example, the intercepttracking log may comprise a bitmap, and the indicators may comprisebits. It may be appreciated that the intercept tracking log may compriseany other data structure or object configured to store data, such as alog. In an example, a single intercept tracking log may be used for thefirst consistency group or individual intercept tracking logs may beused for each file or LUN within the first consistency group. In anexample, shared write functionality may be implemented. For example, theclient write request and a log write to the intercept tracking log maybe performed together. In this way, the implementation of client writerequests and the logging on client write requests to the intercepttracking log may occur together as single operations.

The one or more splitters may continue to split incoming client writerequests during logging. For example, an incoming client write request,targeting a portion of the first consistency group (e.g., a file, a LUN,an entirety of the first consistency group or merely a subset of thefirst consistency group, etc.), may be received. The incoming clientwrite request may be split to create a replicated client write requestbased upon the synchronous replication relationship. The first storagecontroller may be instructed to locally implement the incoming clientwrite request upon the portion of the first consistency group.Accordingly, the incoming client write request may be logged into theintercept tracking log to indicate that the portion of the firstconsistency group has been modified (e.g., that the portion is a dirtyportion that may potentially comprise data that differs from data withina corresponding portion the second consistency group). The secondstorage controller may be instructed to remotely implement thereplicated client write request upon the corresponding portion of thesecond consistency group. Responsive to the incoming client writerequest being logged, the incoming client write request successfullyupdating the portion of the first consistency group, and/or thereplication incoming client write request successfully updating thecorresponding portion of the second consistency group, the incomingclient write request may be acknowledge (e.g., acknowledged as completeback to a client that sent the incoming client write request).

At 306, a first drain without hold operation may be performed to draininflight client write requests occurring before the first point in time(e.g., occurring before the first epoch time). New inflight client writerequests occurring after the first point in time are not held anddrained, but may be normally processed. At 308, a capture of a firstpseudo common snapshot of the second consistency group at the secondstorage controller may be triggered without pausing incoming clientwrite requests. New incoming client write requests are not pausedbecause differences caused by committing the new incoming client writerequests may be captured within the intercept tracking log, and thusclient data access to the first consistency group is not interrupted bythe creation of the first pseudo common snapshot.

At 310, a second drain without hold operation may be performed to draininflight client write requests occurring before the creation of thefirst pseudo common snapshot. For example, the first epoch time may bechanged to a second epoch time, and inflight write requests occurringbefore the second epoch time may be drained (e.g., completed). While thesecond drain without hold operation is being performed, new incomingclient write requests, occurring after the second epoch time, areparallel split and implemented as normal. In this way, inflight writerequests that are completed by the second storage controller but not yetcompleted by the first storage controller due to parallel splitting maybe completed by the first storage controller and captured within theintercept tracking log when a snapshot operation is performed by thefirst storage controller.

At 312, a second pseudo common snapshot of the first consistency groupmay be created at the first storage controller without pausing incomingclient write requests. Not pausing incoming client write requests mayreduce client data access disruption to the first consistency group thatwould otherwise occur if incoming client write requests were held. Thesecond pseudo common snapshot may capture the intercept tracking log ata state that is indicative of a delta between the first pseudo commonsnapshot and the second pseudo common snapshot. The delta may correspondto potential data differences between the first consistency group andthe second consistency group (e.g., data differences resulting fromincoming client write requests being performed between when the firstpseudo common snapshot and the second pseudo common snapshot werecreated). That is, the intercept tracking log captures a delta that is asuperset of the potential differences.

Responsive to creating the first pseudo common snapshot and the secondpseudo common snapshot, the logging of incoming client write requestsmay be stopped. Once the logging has stopped, a third drain without holdoperation may be performed to drain inflight client write operations sothat when the intercept tracking log is deleted, there are no inflightclient write operations attempting to write to the deleted intercepttracking log, which may cause errors. In this way, the intercepttracking log may be deleted after the third drain without hold operationis complete.

The first and second pseudo common snapshots may be used to perform aforward resync in the event the second storage controller falls behindthe first storage controller such as in terms of comprising up-to-datedata (e.g., incoming client write request are completed by the firststorage controller, are but not replicated to the second storagecontroller, such as due to a network issue). For example, thesynchronous replication relationship may be determined as havingtransitioned into an out-of-sync state (e.g., the first consistencygroup comprises more up-to-date data). A local rollback base snapshotmay be created. The local rollback base snapshot may be used to preservean active file system of the second storage controller, such as in theevent a roll back needs to be performed (e.g., to roll back from anerror occurring during the forward resync).

The active file system of the second storage controller may be rolledback based upon differences between the local rollback base snapshot andthe first pseudo common snapshot (e.g., content of the snapshots may bediffed, and the difference may be applied to the active file system). Inthis way, the active file system of the second storage controller isrolled back from the local rollback base snapshot to the first pseudocommon snapshot. Data differences, identified within the intercepttracking log captured within the second pseudo common snapshot, areapplied to the second consistency group of the rolled back active filesystem of the second storage controller until the second consistencygroup mirrors the first consistency group as reflected by the secondpseudo common snapshot (e.g., dirty data is transferred from the secondpseudo common snapshot at the first storage controller to the secondstorage controller for application to the active file system). Forexample, the intercept tracking log, captured within the second pseudocommon snapshot, is evaluated to identify a delta (e.g., a dirtyregion). Dirty data of the delta is transferred from the first storagecontroller to the second storage controller to apply to the secondconsistency group. In this way, the active file system of the secondstorage controller reflects the second pseudo common snapshot.

In an example, a common snapshot of the second consistency group may becreated after the data differences are applied. A resynchronization(e.g., an async engine that utilizes incremental asynchronousreplication and snapshots to transfer data differences from the firststorage controller to the second storage controller) may be performedbetween the first consistency group and the second consistency groupusing the common snapshot and/or one or more incremental snapshots andtransfers. In this way, the synchronous replication relationship may berestored and the first and second consistency groups may be dataconsistent.

The first and second pseudo common snapshots may be used to perform areverse resync in the event the first storage controller falls behindthe second storage controller such as in terms of comprising up-to-datedata (e.g., the first storage controller may fail, and thus the secondstorage controller may perform a switchover operation to provide clientswith failover access to replicated data such as access to the secondconsistency group, and thus the second storage controller has moreup-to-date data). A determination may be made that a switchbackoperation can be performed to give control back from the second storagecontroller to the first storage controller to provide clients withprimary access to data. The switchback operation may be performed, suchas after resynchronization, in response to the first storage controllerrecovering from a failure where the second storage controller performeda switchover operation to provide clients with failover access to thedata previously accessible through the first storage controller.

A local rollback base snapshot may be created. The local rollback basesnapshot may be used to preserve an active file system of the firststorage controller, such as in the event a roll back needs to beperformed (e.g., to roll back from an error occurring during the forwardresync). The active file system of the first storage controller isrolled back based upon the second pseudo common snapshot. The firstconsistency group is restored based upon the first pseudo commonsnapshot until the first consistency group mirrors the secondconsistency group as reflected by the first pseudo common snapshot(e.g., a data transfer is performed from the second storage controllerto the first storage controller to restore the first storage controllerback to the first pseudo common snapshot). For example, the firststorage controller is queried by the second storage controller toidentify data differences recorded within the intercept tracking logmaintained by the first storage controller. The data differences aresent to the first storage controller for overwriting correspondingportions within the first consistency group. In this way, new data maybe transferred from the second storage controller to the first storagecontroller.

In an example, a common snapshot of the second consistency group may becreated after the data differences are applied. A resynchronization maybe performed between the first consistency group and the secondconsistency group using the common snapshot and/or one or moreincremental snapshots and transfers. In this way, the synchronousreplication relationship may be restored and the first and secondconsistency groups may be data consistent.

FIGS. 4A-4H illustrate examples of a system 400 for resynchronization.FIG. 4A illustrates a first storage controller 404 having a synchronousreplication relationship 418 with a second storage controller 410. Thefirst storage controller 404 and the second storage controller 410 maybe configured to communicate with one another over a network 408. In anexample, the synchronous replication relationship 418 may be specifiedbetween a first consistency group 406 (e.g., one or more files and/orLUNs within a volume or spanning multiple volumes hosted by the firststorage controller 404) and a second consistency group 412 (e.g., one ormore files and/or LUNs within a volume or spanning multiple volumeshosted by the second storage controller 410) that is a backupreplication of the first consistency group 406. Accordingly, a writeoperation 402, targeting the first consistency group 406, may beintercepted. A splitter 414 for the first consistency group 406 maylocally implement 416 the write operation 402 upon the first consistencygroup 406, and may send a copy of the write operation 402 to the secondstorage controller 410 for remote implementation 420 upon the secondconsistency group 412. Responsive to both the local implementation 416and the remote implementation 420 successfully completing, anacknowledgement may be provided back to a client that sent the writeoperation 402.

FIG. 4B illustrates a request 426, to create pseudo common snapshots,being received. Accordingly, logging, of incoming client write requests,to an intercept tracking log 428 is enabled. FIG. 4C illustrates loggingof an incoming client write request 432. For example, the incomingclient write request 432, targeting the first consistency group 406, maybe intercepted by the splitter 414 at the first storage controller 404.The splitter 414 may split the incoming client write request 432 into areplicated client write request that is sent to the second storagecontroller 410 for remote implementation 438 upon the second consistencygroup 412. The incoming client write request 432 may be logged 436 intothe intercept tracking log 428, and may be locally implemented 434 uponthe first consistency group 406.

FIG. 4D illustrates a first drain without hold operation 442 beingperformed to drain inflight client write requests occurring before afirst point in time (e.g., an epoch time may be changed to a new epochtime T1, such that inflight client write requests occurring before thenew epoch time T1 may be drained and completed). While the first drainwithout hold operation 442 is being performed, new incoming client writerequests, occurring after the first point in time are allowed to proceednormally. The new incoming client write requests are not affected ordelayed by the first drain with hold operation 442.

FIG. 4E illustrates a first pseudo common snapshot 446 being captured bythe second storage controller 410 without pausing incoming client writerequests to the first storage controller 404 and/or the second storagecontroller 410. The first pseudo common snapshot 446 may capture a pointin time representation of the second consistency group 412 (e.g., asecond volume comprising the second consistency group 412).

FIG. 4F illustrates a second drain without hold operation 450 beingperformed to drain inflight client write requests occurring before thecreation of the first pseudo common snapshot (e.g., the epoch may bechanged to a new epoch time T2, such that inflight client write requestsoccurring before the new epoch time T2 may be drained and completed).While the second drain without hold operation 450 is being performed,new incoming client write requests, occurring after the new epoch timeT2, are parallel split and implemented as normal.

FIG. 4G illustrates a second pseudo common snapshot 456 being capturedby the first storage controller 404 without pausing incoming clientwrite requests to the first storage controller 404 and/or the secondstorage controller 410. The second pseudo common snapshot 456 maycapture a point in time representation of the first consistency group406 (e.g., a first volume comprising the first consistency group 406).

FIG. 4H illustrates the logging of incoming client write requests beingstopped. Once the logging has stopped, a third drain without holdoperation 460 may be performed to drain inflight client write operationsso that when the intercept tracking log 428 is deleted 462, there are noinflight client write operations attempting to write to the deletedintercept tracking log 428, which may otherwise cause errors. In thisway, the intercept tracking log 428 may be deleted 462 after the thirddrain without hold operation 460 is complete.

FIGS. 5A-5F illustrate examples of a system 500 for forwardresynchronization. FIG. 5A illustrates a first storage controller 502having a synchronous replication relationship 512 with a second storagecontroller 508. The first storage controller 502 and the second storagecontroller 508 may be configured to communicate with one another over anetwork 506. In an example, the synchronous replication relationship 512may be specified between a first consistency group 504 (e.g., one ormore files and/or LUNs within a volume or spanning multiple volumeshosted by the first storage controller 502) and a second consistencygroup 510 (e.g., one or more files and/or LUNs within a volume orspanning multiple volumes hosted by the second storage controller 508)that is a backup replication of the first consistency group 504. Thesecond consistency group 510 may be hosted by an active file system 501of the second storage controller 508.

A first pseudo common snapshot 516, of the second consistency group 510,may have been captured by the second storage controller 508. A secondpseudo common snapshot 514, of the first consistency group 504, may havebeen captured by the first storage controller 504. The second pseudocommon snapshot 514 may capture an intercept tracking log 513 at a statethat is indicative of a delta between the first pseudo common snapshot516 and the second pseudo common snapshot 514. The delta may correspondto a data difference between the first consistency group 504 and thesecond consistency group 510 as reflected between the first pseudocommon snapshot 516 and the second pseudo common snapshot 514 (e.g.,deltas resulting from client write requests processed between when thefirst pseudo common snapshot 516 was captured and when the second pseudocommon snapshot 514 was subsequently captured).

FIG. 5B illustrates the synchronous replication relationship 512transitioning into an out-of-sync state 502. For example, connectivitybetween the first storage controller 502 and the second storagecontroller over the network 506 may be interrupted, such that the firststorage controller 502 continues processing incoming client writerequests without replicating the incoming client write requests to thesecond storage controller 508. Accordingly, the second consistency group510 may fall behind the first consistency group 504 because the firstconsistency group 504 may comprise more up-to-date data.

FIG. 5C illustrates a local rollback base snapshot 532 of the activefile system 501 being created. The local rollback base snapshot 532 maycorrespond to a point in time representation of the active file system501, such as the second consistency group 510. FIG. 5D illustrates theactive file system 501 being rolled back 540 based upon a differencebetween the local rollback base snapshot 532 and the first pseudo commonsnapshot 516. FIG. 5E illustrates data differences 560, identifiedwithin the intercept tracking log 513 captured within the second pseudocommon snapshot 514, being applied 540 to the second consistency group510 until the second consistency group 510 mirrors the first consistencygroup 504 as captured by the second pseudo common snapshot 514. Forexample, the first storage controller 502 may send dirty data, as thedata differences 560, from the second pseudo common snapshot 514 to thesecond storage controller 508 for applying 540 to the second consistencygroup 510. FIG. 5F illustrates an asynchronous replication 560 beingperformed to synchronize the second consistency group 510 with a currentstate of the first consistency group 504 (e.g., incremental snapshots ofthe first consistency group 504, such as of a volume and/or active filesystem hosting the first consistency group 504, may be used to performincremental transfers to the second storage controller 508 for updatingthe second consistency group 510).

FIGS. 6A-6G illustrate examples of a system 600 for reverseresynchronization. FIG. 6A illustrates a first storage controller 602having a synchronous replication relationship 612 with a second storagecontroller 608. The first storage controller 602 and the second storagecontroller 608 may be configured to communicate with one another over anetwork 606. In an example, the synchronous replication relationship 612may be specified between a first consistency group 604 (e.g., one ormore files and/or LUNs within a volume or spanning multiple volumeshosted by the first storage controller 602) and a second consistencygroup 615 (e.g., one or more files and/or LUNs within a volume orspanning multiple volumes hosted by the second storage controller 608)that is a backup replication of the first consistency group 604. Thesecond consistency group 615 may be hosted by an active file system 601of the second storage controller 608.

A first pseudo common snapshot 616, of the second consistency group 615,may have been captured by the second storage controller 608. A secondpseudo common snapshot 614, of the first consistency group 604, may havebeen captured by the first storage controller 602. The first pseudocommon snapshot 614 may capture a first intercept tracking log at astate that is indicative of a delta between when the first pseudo commonsnapshot 616 was captured and when the second pseudo common snapshot 614was captured. The delta may correspond to a data difference between thefirst consistency group 604 and the second consistency group 615 asreflected between the first pseudo common snapshot 616 and the secondpseudo common snapshot 614 (e.g., deltas resulting from client writerequests processed between the capture of the first pseudo commonsnapshot 616 and the second pseudo common snapshot 614).

FIG. 6B illustrates the first storage controller 602 experiencing afailure 620. Accordingly, the second storage controller 608 may performa switchover operation 622 to obtain ownership of storage devicespreviously owned by the first storage controller 602 (e.g., ownership ofa storage device comprising the second consistency group 615). In thisway, the second storage controller 608 may provide clients with failoveraccess to the storage device, such as to the second consistency group615 that is a replication of the first consistency group 604. Becausethe second consistency group 615 may be modified without suchmodifications being replicated to the first consistency group 604 due tothe failure 620, the first consistency group 604 may fall behind thesecond consistency group 615 because the second consistency group 615may comprise more up-to-date data. Accordingly, the synchronousreplication relationship 612 may transition into an out-of-sync state.FIG. 6C illustrates the first storage controller 602 recovering from thefailure 602.

FIG. 6D illustrates a local rollback base snapshot 642 of an active filesystem 603 of the first storage controller 602 being created. The localrollback base snapshot 642 may correspond to a point in timerepresentation of the active file system 603, such as the firstconsistency group 604. FIG. 6E illustrates the active file system 603being rolled back 650 based upon the second pseudo common snapshot 614.FIG. 6F illustrates the first consistency group 604 being restored 660based upon the first pseudo common snapshot 616 until the firstconsistency group 604 mirrors the second consistency group 615 asreflected by the first pseudo common snapshot 616 (e.g., a data transferof dirty data 662, within dirty regions 661 specified by the firststorage controller 602 to the second storage controller 608, isperformed by the second storage controller 608 to restore the firststorage controller 602 back to the first pseudo common snapshot 616).For example, the first storage controller 602 is queried by the secondstorage controller 608 to identify data differences recorded within theintercept tracking log maintained by the first storage controller 602.The data differences are sent to the first storage controller 602 foroverwriting corresponding portions within the first consistency group604. In this way, dirty data 662, corresponding to the dirty regions661, may be transferred from the second storage controller 608 to thefirst storage controller 602.

FIG. 6G illustrates an asynchronous replication 670 being performed tosynchronize the first consistency group 604 with a current state of thesecond consistency group 615 (e.g., incremental snapshots of the secondconsistency group 615, such as of a volume and/or active file systemhosting the second consistency group 615, may be used to performincremental transfers to the first storage controller 602 for updatingthe first consistency group 604).

Still another embodiment involves a computer-readable medium comprisingprocessor-executable instructions configured to implement one or more ofthe techniques presented herein. An example embodiment of acomputer-readable medium or a computer-readable device that is devisedin these ways is illustrated in FIG. 7, wherein the implementation 700comprises a computer-readable medium 708, such as a CD-ft DVD-R, flashdrive, a platter of a hard disk drive, etc., on which is encodedcomputer-readable data 706. This computer-readable data 706, such asbinary data comprising at least one of a zero or a one, in turncomprises a processor-executable computer instructions 704 configured tooperate according to one or more of the principles set forth herein. Insome embodiments, the processor-executable computer instructions 704 areconfigured to perform a method 702, such as at least some of theexemplary method 300 of FIG. 3, for example. In some embodiments, theprocessor-executable computer instructions 704 are configured toimplement a system, such as at least some of the exemplary system 400 ofFIGS. 4A-4H, at least some of the exemplary system 500 of FIGS. 5A-5F,and/or at least some of the exemplary system 600 of FIGS. 6A-6G, forexample. Many such computer-readable media are contemplated to operatein accordance with the techniques presented herein.

It will be appreciated that processes, architectures and/or proceduresdescribed herein can be implemented in hardware, firmware and/orsoftware. It will also be appreciated that the provisions set forthherein may apply to any type of special-purpose computer (e.g., filehost, storage server and/or storage serving appliance) and/orgeneral-purpose computer, including a standalone computer or portionthereof, embodied as or including a storage system. Moreover, theteachings herein can be configured to a variety of storage systemarchitectures including, but not limited to, a network-attached storageenvironment and/or a storage area network and disk assembly directlyattached to a client or host computer. Storage system should thereforebe taken broadly to include such arrangements in addition to anysubsystems configured to perform a storage function and associated withother equipment or systems.

In some embodiments, methods described and/or illustrated in thisdisclosure may be realized in whole or in part on computer-readablemedia. Computer readable media can include processor-executableinstructions configured to implement one or more of the methodspresented herein, and may include any mechanism for storing this datathat can be thereafter read by a computer system. Examples of computerreadable media include (hard) drives (e.g., accessible via networkattached storage (NAS)), Storage Area Networks (SAN), volatile andnon-volatile memory, such as read-only memory (ROM), random-accessmemory (RAM), EEPROM and/or flash memory, CD-ROMs, CD-Rs, CD-RWs, DVDs,cassettes, magnetic tape, magnetic disk storage, optical or non-opticaldata storage devices and/or any other medium which can be used to storedata.

Although the subject matter has been described in language specific tostructural features or methodological acts, it is to be understood thatthe subject matter defined in the appended claims is not necessarilylimited to the specific features or acts described above. Rather, thespecific features and acts described above are disclosed as exampleforms of implementing at least some of the claims.

Various operations of embodiments are provided herein. The order inwhich some or all of the operations are described should not beconstrued to imply that these operations are necessarily orderdependent. Alternative ordering will be appreciated given the benefit ofthis description. Further, it will be understood that not all operationsare necessarily present in each embodiment provided herein. Also, itwill be understood that not all operations are necessary in someembodiments.

Furthermore, the claimed subject matter is implemented as a method,apparatus, or article of manufacture using standard application orengineering techniques to produce software, firmware, hardware, or anycombination thereof to control a computer to implement the disclosedsubject matter. The term “article of manufacture” as used herein isintended to encompass a computer application accessible from anycomputer-readable device, carrier, or media. Of course, manymodifications may be made to this configuration without departing fromthe scope or spirit of the claimed subject matter.

As used in this application, the terms “component”, “module,” “system”,“interface”, and the like are generally intended to refer to acomputer-related entity, either hardware, a combination of hardware andsoftware, software, or software in execution. For example, a componentincludes a process running on a processor, a processor, an object, anexecutable, a thread of execution, an application, or a computer. By wayof illustration, both an application running on a controller and thecontroller can be a component. One or more components residing within aprocess or thread of execution and a component may be localized on onecomputer or distributed between two or more computers.

Moreover, “exemplary” is used herein to mean serving as an example,instance, illustration, etc., and not necessarily as advantageous. Asused in this application, “or” is intended to mean an inclusive “or”rather than an exclusive “or”. In addition, “a” and “an” as used in thisapplication are generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Also, at least one of A and B and/or the like generally means A orB and/or both A and B. Furthermore, to the extent that “includes”,“having”, “has”, “with”, or variants thereof are used, such terms areintended to be inclusive in a manner similar to the term “comprising”.

Many modifications may be made to the instant disclosure withoutdeparting from the scope or spirit of the claimed subject matter. Unlessspecified otherwise, “first,” “second,” or the like are not intended toimply a temporal aspect, a spatial aspect, an ordering, etc. Rather,such terms are merely used as identifiers, names, etc. for features,elements, items, etc. For example, a first set of information and asecond set of information generally correspond to set of information Aand set of information B or two different or two identical sets ofinformation or the same set of information.

Also, although the disclosure has been shown and described with respectto one or more implementations, equivalent alterations and modificationswill occur to others skilled in the art based upon a reading andunderstanding of this specification and the annexed drawings. Thedisclosure includes all such modifications and alterations and islimited only by the scope of the following claims. In particular regardto the various functions performed by the above described components(e.g., elements, resources, etc.), the terms used to describe suchcomponents are intended to correspond, unless otherwise indicated, toany component which performs the specified function of the describedcomponent (e.g., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure. In addition, while aparticular feature of the disclosure may have been disclosed withrespect to only one of several implementations, such feature may becombined with one or more other features of the other implementations asmay be desired and advantageous for any given or particular application.

What is claimed is:
 1. A method comprising: receiving, by a storageserver, a request to make pseudo common snapshots of a first consistencygroup, hosted by a first storage controller, and a second consistencygroup hosted by a second storage controller, the first consistency groupand the second consistency group having a synchronous replicationrelationship; enabling logging, of incoming client write requests, to anintercept tracking log at a first point in time, the intercept trackinglog comprising indicators set to indicate whether portions of the firstconsistency group have been modified or not modified with respect to thesecond consistency group; performing a first drain without holdoperation to drain inflight client write requests occurring before thefirst point in time; triggering a capture of a first pseudo commonsnapshot of the second consistency group at the second storagecontroller without pausing incoming client write requests; performing asecond drain without hold operation to drain inflight client writerequests occurring before the creation of the first pseudo commonsnapshot; and creating a second pseudo common snapshot of the firstconsistency group at the first storage controller without pausingincoming client write requests, the second pseudo common snapshotcapturing the intercept tracking log at a state that is indicative of adelta between the first pseudo common snapshot and the second pseudocommon snapshot, the delta corresponding to data differences between thefirst consistency group and the second consistency group as reflectedbetween the first pseudo common snapshot and the second pseudo commonsnapshot.
 2. The method of claim 1, comprising: responsive to creatingthe first pseudo common snapshot and the second pseudo common snapshot,stopping the logging of incoming client write requests.
 3. The method ofclaim 2, comprising: responsive to stopping the logging of incomingclient write requests, performing a third drain without hold operationto drain inflight client write requests.
 4. The method of claim 3,comprising: responsive to completing the third drain without holdoperation, deleting the intercept tracking log.
 5. The method of claim1, wherein the enabling logging comprises: receiving an incoming clientwrite request targeting a portion of the first consistency group;splitting the incoming client write request to create a replicatedclient write request based upon the synchronous replicationrelationship; instructing the first storage controller to locallyimplement the incoming client write request upon the portion of thefirst consistency group; logging the incoming client write request intothe intercept tracking log to indicate that the portion of the firstconsistency group has been modified; instructing the second storagecontroller to remotely implement the replicated client write requestupon a corresponding portion the second consistency group; andresponsive to the incoming client write request being logged, theincoming client write request successfully updating the portion of thefirst consistency group, and the replication incoming client writerequest successfully updating the corresponding portion of the secondconsistency group, acknowledging the incoming client write request ascomplete.
 6. The method of claim 1, comprising: determining that thesynchronous replication relationship has transitioned into anout-of-sync state; creating a local rollback base snapshot; rolling backan active file system of the second storage controller based upondifferences between the local rollback base snapshot and the firstpseudo common snapshot; and applying the data differences, identifiedwithin the intercept tracking log captured within the second pseudocommon snapshot, to the second consistency group until the secondconsistency group mirrors the first consistency group.
 7. The method ofclaim 6, wherein the applying the data differences comprises: evaluatingthe intercept tracking log to identify a delta; and transferring dirtydata of the delta from the first storage controller to the secondstorage controller to apply to the second consistency group.
 8. Themethod of claim 6, comprising: creating a common snapshot of the secondconsistency group after the data differences are applied; and performinga resynchronization between the first consistency group and the secondconsistency group using the common snapshot and one or more incrementalsnapshots.
 9. The method of claim 1, comprising: determining that thefirst storage controller has recovered from a failure where the secondstorage controller performed a switchover operation to provide clientswith failover access to the data previously accessible through the firststorage controller; creating a local rollback base snapshot; rollingback an active file system of the first storage controller based uponthe second pseudo common snapshot; and restoring the first consistencygroup based upon the first pseudo common snapshot until the firstconsistency group mirrors the second consistency group as reflected bythe first pseudo common snapshot.
 10. The method of claim 9, wherein therestoring the second consistency group comprises: identifying datadifferences between the first consistency group and the secondconsistency group; and sending the data differences to the first storagecontroller for overwriting corresponding portions within the firstconsistency group.
 11. The method of claim 9, comprising: creating acommon snapshot of the second consistency group after the firstconsistency group is restored; and performing a resynchronizationbetween the first consistency group and the second consistency groupusing the common snapshot.
 12. The method of claim 1, wherein theintercept tracking log comprises a bitmap, and the indicators comprisebits.
 13. The method of claim 1, wherein the consistency group comprisesat least one of one or more files, one or more directories, a volume, orone or more logical unit numbers (LUNs).
 14. The method of claim 1,comprising: receiving a metadata operation while logging to theintercept tracking log is enabled; preserving the metadata operation asa record; and utilizing the record during a resynchronization betweenthe first consistency group and the second consistency group using thefirst pseudo common snapshot and the second pseudo common snapshot. 15.The method of claim 14, wherein the metadata operation corresponds to atleast one of a file size change command, a LUN size change command, aset attribute command, a delete command, a create command, or a modifycommand.
 16. A non-transitory machine readable storage medium havingstored thereon instructions for performing a method comprising machineexecutable code which when executed by at least one machine, causes themachine to: determine that a synchronous replication relationship,between a first consistency group hosted by a first storage controllerand a second consistency group hosted by a second storage controller,has transitioned into an out-of-sync state where a delta exists betweenthe first consistency group and the second consistency group; create alocal rollback base snapshot; roll back an active file system of thesecond storage controller based upon differences between the localrollback base snapshot and a first pseudo common snapshot correspondingto a first point in time representation of the second consistency group;and apply data differences, identified within an intercept tracking logof a second pseudo common snapshot corresponding to a second point intime representation of the first consistency group, to the secondconsistency group until the second consistency group mirrors the firstconsistency group.
 17. The non-transitory machine readable storagemedium of claim 16, wherein the machine executable code causes themachine to: evaluate the intercept tracking log to identify the delta;and transfer dirty data of the delta from the first storage controllerto the second storage controller to apply to the second consistencygroup.
 18. The non-transitory machine readable storage medium of claim16, wherein the machine executable code causes the machine to: create acommon snapshot of the second consistency group after the datadifferences are applied; and perform a resynchronization between thefirst consistency group and the second consistency group using thecommon snapshot and one or more incremental snapshots.
 19. A computingdevice comprising: a memory containing machine readable mediumcomprising machine executable code having stored thereon instructionsfor performing a method of resynchronization; and a processor coupled tothe memory, the processor configured to execute the machine executablecode to cause the processor to: determine that a first storagecontroller has recovered from a failure where a second storagecontroller performed a switchover operation to provide clients withfailover access to the data previously accessible through the firststorage controller; create a local rollback base snapshot; roll back anactive file system of the first storage controller based upon a secondpseudo common snapshot corresponding to a first point in timerepresentation of a first consistency group; and restore the firstconsistency group based upon a first pseudo common snapshot,corresponding to a second point in time representation of a secondconsistency group, until the first consistency group mirrors the secondconsistency group as reflected by the first pseudo common snapshot. 20.The computing device of claim 19, wherein the machine executable codecauses the processor to: create a common snapshot of the secondconsistency group after the first consistency group is restored; andperform a resynchronization between the first consistency group and thesecond consistency group using the common snapshot.